Effect of insulating liquid on thrust force of a synchronous electrostatic film actuator

Electrostatic film actuators, which are flexible and light-weight linear motors, can possibly realize safe contact between the robot and environments, if adopted for robots. However, the electrostatic film motors have suffered from a technical issue caused by insulating liquid. To generate large force, the actuator must be immersed in insulating liquid. Lower viscosity has been preferred for the liquid, but such liquid can more easily volatilize and causes maintenance problems. To utilize highly-viscous liquid with lower volatility, this paper investigates how highly-viscous liquid affects the thrust force performance. Three silicone oils with different viscosities, as well as one insulating liquid (Fluorinert FC-770 (3M)), were compared in several different conditions. From our analysis, although high viscosity can degrades the force performance due to drag, the results suggested that the highly-viscous oil in the gap was moved by electrostatic force, and thus the viscosity does not always degrade the thrust force. It was also confirmed that in some particular conditions, the use of highly-viscous oil can even enhance the thrust force. On the other hand, highly-viscous oil degrades the backdrivability of the actuator while it is at rest, meaning that there is a trade-off between the force enhancement, volatility, and backdrivability.